# TRACES: FR-DEV-3a | FR-DEV-3c
id: exposure
label: op.exposure
order: 20
attributes: [tone]

doc: |
  Exposure — a linear gain, expressed in stops.

  The simplest operation in the pipeline and the one that most justifies
  working in linear light: a stop is a doubling, so exposure is a single
  multiply. Applied to gamma-encoded data it would be neither a doubling nor
  reversible, which is why this stage sits where it does (ARCH §5.2).

placement: |
  A correction to how the scene was captured, so it precedes every operation
  that interprets tone.

params:
  exposure:
    label: param.exposure
    kind: stops
    min: -5
    max: 5
    doc: |
      ±5 stops. Wider than most edits need, but recovering a badly
      underexposed frame is a real use and raw data often supports it.

uniforms:
  gain:
    value: exp2(exposure)
    doc: The linear gain for the current setting.

wgsl: |
  c = c * gain;

tests:
  - name: one_stop_is_a_doubling
    why: |
      The definition of a stop. If this is wrong, every exposure adjustment is
      subtly off and no test of "looks right" would catch it.
    set: { exposure: 1 }
    expect: { gain: 2.0 }

  - name: one_stop_down_is_a_halving
    set: { exposure: -1 }
    expect: { gain: 0.5 }

  - name: stops_compose_additively
    why: +2 stops must equal +1 applied twice.
    set: { exposure: 2 }
    expect: { gain: 4.0 }

  - name: neutral_is_unity_gain
    why: |
      An unedited image must be the image, not an interpretation of it.
    expect: { gain: 1.0 }
    expect_active: false